1. Field of the Invention
Ammonium sulfate is an important nitrogen-sulfur fertilizer in United States and world agriculture. It is produced as a by-product of coke or caprolactam production as well as by ammoniation of spent sulfuric acid. By-product ammonium sulfate supply in the United States has remained relatively constant over the past several years at approximately 2,000,000 tons per year. The utilization of ammonium sulfate as a fertilizer is oftentimes advantageous since it contains both nitrogen and sulfur in readily available forms and it is strongly acid forming, a benefit when applied to alkaline soils. Furthermore, it is a source of ammoniacal nitrogen not vulnerable to denitrification and it also is a biuret-free nitrogen source eminently useful for citrus fertilization. These advantages, as well as others, have caused ammonium sulfate to be viewed as an economical nitrogen and sulfur source compared to substitute products. Furthermore, alternate sources of fertilizer sulfur such as commonly used ammonium thiosulfate (ATS) solution and elemental sulfur (ES) are much more costly than the low-quality, small-crystal variety of by-product ammonium sulfate. Substitution of this low-quality, small-crystal form of ammonium sulfate for ATS solution or ES could provide truly remarkable savings and increased profits to fertilizer dealers by greatly reducing their raw material costs for both fertilizer sulfur and nitrogen. In the majority of cases the savings range from $75 to over $100 for each ton of ATS solution and over $200 for each ton of ES suspension replaced with the AS fluids of the present invention. This would represent a very large savings in the fertilizer industry, where only a few dollars per ton is considered a substantial difference in price.
However, much of the by-product AS on the market today is most unsatisfactory for storage or use because the by-product AS crystals are too small and/or contain too much moisture. To be satisfactory for storage and use, AS must ordinarily be produced and shipped in the form of large, relatively dry crystals or granules. Small AS crystals pack and cake severely during shipping and storage, making the material quite difficult or impossible to handle, mix, or apply as a solid. Incidences of entire storage bins of this low-quality AS rapidly becoming solidified or essentially one solid piece have been reported, precluding use of the material as well as removal of the material to make room for other fertilizer materials. Use of dynamite to break up the solidified material has been reported. These small, often wet AS crystals have been observed to essentially fuse together into an almost ceramic state. If the material can be broken up, it must be carefully screened to remove large lumps which remain and complicate blending and application of the material. The larger AS crystals or granules are considered more suitable for bulk blending and application because they are more well matched in size with other fertilizer solids normally used in bulk blending and application in solid form; the small crystals are unsuitable for this purpose because segregation and uneven application occurs with fertilizer mixtures containing widely varying particle sizes (see Hoffmeister, George. "Quality Control in a Bulk Blending Plant," Proc. TVA Fertilizer Bulk Blending Conference, Louisville, Ky., Aug. 1-2, 1973). However, the large-crystal and granular forms of AS are considerably more expensive (2 to 8 times as much depending on location) than the low-quality, small-crystal variety, because processing costs are considerably higher for granulating the AS or for producing large AS crystals and because the larger-sized varieties simply bring higher prices on the market because of the demand for their higher quality. Granulation of ammonium sulfate is a complex and expensive operation which requires large production rates to be profitable. Also, production of large AS crystals requires complex and expensive crystallizer systems with long crystallizer retention times as well as costly centrifuging and drying operations (see "Ammonium Salts, Nitric Acid, and Nitrates," Fertilizer Manual, International Fertilizer Development Center, Reference Manual IFDC-R-1, Published December 1979, Chapter VIII, pages 83-85). On the other hand, the small and often wet crystal varieties of AS such as the by-product of coke production or the AS fines portion of other AS production operations, are considerably less expensive than the granular or large crystal varieties, mainly because of the low demand on the market for AS with such poor properties. In addition, these small-crystal, low-quality forms of AS are the products of simple and relatively crude crystallization systems employed in low production rate applications or in other situations where the more elaborate and expensive processing systems required for production of granules or large crystals are either unaffordable or otherwise not practical or desirable. For example, in production of coke, which is used as a carbon source in steel production, coal is heated in ovens, resulting in formation of the coke and a coke gas containing ammonia. The ammonia in the coke gas must be removed early in the process for corrosion considerations. The ammonia is removed by scrubbing the coke gas with sulfuric acid and the effluent from the scrubber is ammonium sulfate slurry. This ammonium sulfate slurry is then concentrated with respect to the solids content and then simply sent to a centrifuge or other equipment to remove the ammonium sulfate liquid from the ammonium sulfate crystals. Of course, the slurry, if not treated, will quickly deposit the solids content thereof onto the bottom of storage vessels thereby rendering same totally useless. Thus, the AS slurry must be subjected to crystal removal operations shortly after it is produced. Obviously, it cannot be shipped or stored as made. The crystals exit the centrifuge small and wet and are sent to storage in this condition without a drying step, exhibiting all of the aforementioned problems with these types of crystals. Drying the ammonium sulfate crystals improves storage of the small crystals somewhat but the drying step is expensive, and even the dried fine-crystal AS is still quite unsatisfactory for use in the solid fertilizer industry because it is not well matched in size with other fertilizer solids and still tends to pack and cake. Larger ammonium sulfate producers such as in the caprolactam industry have expensive, elaborate, and large crystallizers and use same with long retention times in an effort to produce large crystals. They also have to use rather expensive centrifuges, and expensive driers in an effort to improve product storage properties (see Reference Manual IFDC-R-1). These types of operations result in formation of larger, drier crystals with improved storage properties but at greater cost for production; however, even these larger, drier crystals are often too small to be used satisfactorily in production of solid fertilizer blends.
Although these small-crystal, low-quality, inexpensive forms of AS are quite unsuitable for storage, shipping, use, and application as a solid, it has now been discovered that they are ideal for production and application in suspension form. More succintly, such ammonium sulfate, which is the lowest quality as a solid fertilizer, is of the highest quality when in suspension form, because the production and very existence of a plethora of such small crystals result in suspensions having superior storage, handling, use, and application properties. Since the AS provides a very inexpensive raw material feedstream, production of such AS suspensions proves to be considerably more economical than other commonly available fluid fertilizer sulfur sources, such as ATS solution and ES suspensions, supra. It has now been found that production of AS in suspension form from these low-quality, small-crystal varieties of AS provides a successful and stable means for storage thereof, thereby providing the fertilizer dealer with a good, inexpensive fluid sulfur source immediately ready, when needed during the peak fertilizer rush season, without the complications and time lost in having to first break up and then screen solid AS and/or dissolve same in aqueous media. It has been further found that said crystal-containing AS suspensions can, after long periods of storage, be successfully used in production of crystal-free, true solution fertilizers. Although the logistics of fertilizer operations favor concentrated suspension for storage and shipping, crystal-free solution fertilizer solutions are preferred for application. Solutions can be applied through smaller nozzles at more uniform rates with little or no danger of plugging of the lines or nozzles. Accordingly, the lower concentrations of the crystal-free solutions of the present invention are not an important factor and are more than offset by the ease and uniformity of application of the crystal-free solutions.
Heretofore, compositions and processes of the prior art from which NS fluids are produced, using either ammonium sulfate or a combination of sulfuric acid and ammonia, have for the most part been restricted to being weak solution of low-grade and low-sulfur content because of the relatively low solubility of such ammonium sulfate in water and various other fertilizer fluids. Even with AS in water along, the highest solution grade which can normally be made is 8-0-0-9S. Of course, such low concentrations substantially increase shipping and storage costs per unit of plant food as well as severely limit the degree of flexibility in formulating desired grades and compositions of fluid fertilizer required in the fluid fertilizer industry. In practice of prior-art teachings for production of NS suspensions containing undissolved fertilizer salts, many of the products have contained undesirable solid phases which result in poor storage and handling properties or they have contained other fertilizer compounds, such as urea and/or ammonium nitrate, which substantially increase the cost of production and lower the products' economic advantage over other fertilizer sulfur sources. In either case, the NS suspensions of the teachings of the prior art contain appreciable proportions of undissolved solids at the time of application, which solid phase unduly complicates application of the suspensions to the soil when crystals therein plug spray nozzles or settle to the bottom of tanks, during the vibrations which normally occur during transit. Indeed, the propensity of many suspensions to settle out during transport, as by rail, has been specifically addressed in U.S. Pat. No. 4,066,432, Jones, Jan. 3, 1978, assigned to the assignee of the present invention.
2. Description of the Prior Art
The prior art to date reveals that there are available a number of methods and means which teach the production, in one way or another, of nitrogen-sulfur fluid fertilizers using ammonium sulfate and/or sulfuric acid and ammonia. Some of these prior-art teachings are represented by the investigations, teachings, and disclosures set forth in the following patents: U.S. Pat. No. 4,762,546, Boles, Aug. 9, 1988 (assigned to the assignee of the present invention); U.S. Def. Pub. No. T101,803, Jones et al., May 4, 1982 (assigned to the assignee of the present invention); U.S. Pat. No. 4,116,664, Jones, Sept. 26, 1978; Canadian Patent No. 811,080, Ramaradhya, Apr. 22, 1969; U.S. Pat. No. 4,388,101, Lowder, Jun. 14, 1983; U.S. Pat. No. 4,239,522, Wilson et al., Dec. 16, 1980. Procedures for producing fluid fertilizers containing both nitrogen and sulfur have been developed, since fluid fertilizers containing sulfur are now needed in many regions of the country for soils which are sulfur deficient. One particular procedure for production of a liquid fertilizer containing both nitrogen and sulfur (as in Jones '664, supra) involves reaction of urea with sulfuric acid to form a liquid nitrogen-sulfate fertilizer comprising urea-sulfate and liquefied urea. Sulfuric acid is added gradually to urea, which urea is preferably in powdered or prilled form, and added in controlled amounts to hold the temperature of the resulting reaction within prescribed limits. The combination of sulfuric acid and urea form a resulting reacting molten slurry which is blended slowly during the reaction period. Sulfuric acid is gradually added until the total desired amount thereof has been added, and blending is continued until the slurry becomes completely liquefied. Water is subsequently added to produce desired products which will remain in liquid form at normal ambient temperatures. From the practice of this procedure, a resulting product of grad 31-0-0-9.7S will begin to solidify at a temperature of about 60.degree. F. If this product is diluted with water to a grade of 29-0-0-9S, the then resulting product will begin to solidify at about 10.degree. F. The pH of these products ranges from 0.4 to 1.0.
The practice of another prior-art teaching reportedly also yields nitrogen-sulfur suspension (as in Ramaradhya 811,080, supra). This procedure involves pregelling clay in urea-ammonium nitrate solution (32% N) and incorporating finely divided elemental sulfur in the solution-clay mixture by mixing in a tank with a propeller-type mixer. The grade of the resulting suspension is approximately 24-0-0-23S, and it is reported that the stability of the product is adequate for short-term storage.
Still another method for producing a nitrogen-sulfur suspension taught by Jones, et al. ('803, supra) involves the reaction of sulfuric acid with gaseous ammonia and the simultaneous addition thereto of a urea-water solution in a single-stage reactor to produce a resulting boiling urea-ammonium sulfate solution. The boiling solutions are then rapidly cooled in two stages to about 100.degree. F. to produce therein an abundance of small urea crystals. The finished product is of grade 29-0-0-5S and contains mostly urea as the solid phase. Because of urea's high solubility and highly temperature-dependent solubility, urea crystals, as is generally well known, are subject to rapid growth to large sizes during storage.
In still another reported procedure, Lowder ('101, supra) teaches the production of nitrogen-sulfur solutions by first mixing sulfuric acid in water, followed by dissolving urea into the resulting acid solution, and finally by adding thereto anhydrous ammonia. However, because the products are solutions, in which the highest grades were limited by solubility, they are low in grade (19 to 25% nitrogen and 3 to 6% sulfur) and have rather high crystallization temperatures (32.degree. to 40.degree. F.) below which the products cannot be stored, because the large crystals which form settle to the bottom of storage tanks or plug up solution application equipment. Also, the low concentrations of these solutions substantially increase shipping and storage costs per unit of plant food as well as severely limit the degree of flexibility in formulating desired grades and compositions of fluid fertilizer normally expected in the fluid fertilizer industry with other sulfur sources.
In yet another procedure taught in the prior art, Wilson, et al. ('522, supra) produce nitrogen-sulfur solutions containing urea, ammonium nitrate, and ammonium sulfate. Because these products are solutions, the grades are low relative to suspensions and, because of ammonium sulfate's low solubility in UAN-32 (a urea-ammonium nitrate solution containing 32% nitrogen), the sulfur contents of these products are relatively low unless the nitrogen content thereof is drastically reduced. Here again, the low concentrations of these solutions substantially increase shipping and storage costs per unit of plant food as well as severely limit the degree of flexibility in formulating desired grades and compositions of fluid fertilizer normally expected in the fluid fertilizer industry with other sulfur sources.
In still another procedure taught in the prior art, Boles ('546, supra) produces nitrogen-sulfur suspensions from by-product ammonium sulfate or sulfuric acid and ammonia with addition of other nitrogen fluids or solids containing urea and/or ammonium nitrate. However, because these products contain urea and/or ammonium nitrate, which are relatively high-priced fertilizer compounds compared with small-crystal ammonium sulfate, their economic advantage over other higher priced fertilizer sulfur sources such as elemental sulfur and ammonium thiosulfate solution is reduced. In addition, if appreciable proportions of urea and ammonium nitrate are not added, the product pH is too low, which precludes proper gelation of the suspending clay and results in rapid deterioration of the products' physical and storage properties. Also the NS suspensions produced by Boles contain appreciable proportions of undissolved solids at the time of application which complicates application of the suspensions to the soil when crystals plug spray nozzles or settle to the bottom of application tanks during the vibrations which normally occur during transit to the application site or during the actual application operation.
It will, of course, be appreciated that there is no suggestion in the teachings of any of the above-mentioned, prior-art references of the viable process and/or techniques as well as the compositions resulting from the practice of the instant invention for easily and consistently producing, storing, handling, and using the very inexpensive, low-quality form of by-product ammonium sulfate (AS) which has very poor storage, handling, and use properties and is normally most unsuitable as a solid fertilizer because the crystals are too small and wet. It will be further appreciated that the art does not teach a process for the production of concentrated, high-grade, high-quality, long-storing suspensions containing appreciable AS solids directly from low-cost, impure AS slurries or crystals, by operation of a simple and economical process which can be either of the batch or continuous type, and in which process the addition of very small amounts of ammonia or another suitable base such as potassium hydroxide during production of said suspensions is utilized as a clay stabilizer in substitution for much larger amounts of higher priced-compounds such as urea and/or ammonium nitrate, and wherein the resulting AS suspensions are, or can be, stored as intermediate feed stream materials and further, wherein is produced crystal-free, true solution NS fertilizers from such crystal-containing suspension intermediates and still further, wherein there is eliminated the need for large, elaborate, and costly crystallizer systems, which are normally operated for long periods of time for effecting sufficient size of crystal growth of the resulting AS crystals such that they will be large enough for satisfactory storage and use as solid fertilizer and even still further, wherein elimination of the need for the costly and difficult process steps which are normally required for production of AS crystals dry enough for satisfactory storage and use as solid fertilizer including the centrifuging of the AS slurry to effect separation of the crystals from the mother liquor as well as the final drying of such separated AS crystals.